1. Field of the Invention
This invention relates generally to enhancement of optical components or devices and more particularly to improvements in optical components and devices employed in photonic integrated circuits (PICs) or PIC chips using free space coupler regions.
2. Description of the Related Art
In-based integrated optical components in a monolithic photonic integrated circuit (PIC) chip have become a reality in recent times. Examples of such PIC chips are disclosed in U.S. patent application, Ser. No. 10/267,331, filed Oct. 8, 2002 and U.S. patent application, Ser. No. 10/267,304, filed Oct. 8, 2002, both of which are incorporated herein by their reference. One version of such a PIC is a monolithic optical transmitter photonic integrated circuit, or TxPIC, fabricate din an InP-based alloy system which include an array of modulated laser sources, such as DFB or DBR laser arrays, with their outputs coupled to a wavelength selective combiner such as an arrayed waveguide grating (AWG), or such CW operated laser sources are coupled through a corresponding array of electro-optic modulators to an AWG. The laser sources are designed to each operate at a different wavelength and together form a wavelength grid designed to match a standardized wavelength grid, such as the standard ITU wavelength grid.
The complexities in the manufacture of such TxPIC chips to specified wavelengths and desired wavelength grids is difficult to achieve in a uniform manner providing good reproducibility and high yield. For example, if the passband of the AWG is off or shifted from the desired wavelength grid and the laser source wavelength grid is as exactly as designed, the light from the laser sources may not pass through the AWG or, otherwise, may be severely attenuated from passing through the AWG. In general, if the AWG passband and the laser source wavelengths are not aligned, then an insignificant amount of light will emerge from the TxPIC chip rendering the chip of useless utility. To ease the manufacturing tolerances of the PIC AWG, as taught in Ser. No. 10/267,331, supra, a plurality of vernier outputs are formed at the output of the wavelength selective combiner to the facet exit of the chip. Each vernier output represents a slightly different selection of laser source wavelengths emerging from the AWG. Thus, chances are increased that one of the AWG vernier outputs will optimally align to the laser source wavelength grid relative to the passband of the AWG so that the laser source wavelengths will be substantially matched to at least one of the vernier outputs. As indicated, disclosure of these combiner vernier outputs can be found in Ser. No. 10/267,331, supra.
To test such a TxPIC chip, one approach is to measure the light out of the wavelength selective combiner for each laser source as a function of both applied current to the laser sources and their ambient temperature. If the TxPIC has any chance of utility, there is a temperature and range of currents where the laser wavelength sources will be substantially aligned with at least one of the combiner vernier outputs from the chip. For a discrete TxPIC chip, such testing can be accomplished by employing a large area detector or an integrating sphere. What would be more desirable is if such testing could be accomplished while the PIC chips remain in-wafer, i.e., prior to singulation of PIC die from an as-grown InP wafer, rather than later testing as a discrete PIC die. An advantage is obtained relative to advance knowledge of the PIC component operability and selection of a group of probable vernier outputs where the optimum combiner vernier output may lie or selection of the optimum vernier output exhibiting the highest matching quality of the laser wavelength grid to the passband and wavelength grid of the combiner. Compare this testing of individual die after their singulation which requires additional resources and time to mount the individual chips for such testing followed by individual testing of each chip for operability and optimum vernier output only to discover that the prepared chips are not operative or adequate for use. It would be desirable to know before wafer singulation which PIC die can be discarded because of their noted failure during in-wafer testing. Also, it would be helpful to know before wafer singulation which vernier output or, at least, subgroup of vernier outputs are favored, for the best laser source wavelength grid/combiner passband match prior to wafer singulation.
InP-based wavelength selective combiners, such as, Echelle gratings, arrayed waveguide gratings (AWGs) or cascaded Mach-Zehnder interferometers are of interest for a variety of applications. One of the most interesting of these applications is their deployment in photonic integrated circuits (PICs) as multiplexing and/or demultiplexing components or devices. The successful realization of practical devices utilizing, for example, InP-based AWGs, requires several features which also represent problems to be solved:
1. The ability to environmentally, electrically and optically passivate etched waveguides.
2. The ability to form a polarization insensitive device.
3. The ability to reduce the refractive index step between the waveguide and the free-space region or slab of the AWG for reduced insertion loss.
4. Compatibility with planar PIC processing.
5. The ability to isolate AWGs from active or activating components placed on an AWG or on the PIC in close proximity to an AWG, e.g., on-chip heaters or tuning electrodes.
6. Reduce the effects of side wall surface roughness in etching the AWG waveguide ridge structure.
The conventional technique for accomplishing features 1–6 in the art is to utilize buried structures wherein InP regrowth is utilized to form an overlayer or burying layer. However, buried waveguide structures are difficult to achieve on a reproducible and repeated basis, require sophisticated wafer fabrication and epitaxial growth, result in lower yield, and are generally more costly to manufacture. Ridge waveguide structures are preferred for reasons of simplicity, yield and cost. However, the problems associated with items 1–6 above must be addressed in a rigid waveguide structure in order to realize a practical optical component or device.
Another aspect of PICs utilizing an optical combiner as an integrated component is the design of the component to have low insertion loss (IL). With the increase of the number of components integrated on a single chip, the requirements for wafer uniformity as well as uniformity in layer growth in composition and thickness becomes a more critical issue. One way of lowering insertion losses in the AWG, for example, which is documented in the art, is to reduce the refractive index change in the transition coupling region between the multiple waveguides of the AWG and the free space region of the AWG. An example of this art is shown in the article of J. H. den Besten et al. entitled, “Low-Loss, Compact, and Polarization Independent PHASAR Demultiplexer Fabricated by Using a Double-Etch Process”, IEEE Photonics Technology Letters, Vol. 14(1), pp. 62–64, January, 2002. As shown in this article, shallow and deep etched waveguides are combined such that a widening of the propagating mode is provided from the deep ridge of the waveguide to the shallow ridge of the waveguide and thence to the free space region of the AWG. This provides for a gradual or monotonic and adiabatic expansion of the mode through such a transition region decreasing insertion losses and coupling losses between the waveguide and the free space region as well as improving optical coupling between adjacent waveguides in the transition region and coupled to the free space region. What is desired is to improve the reduction in insertion loss without requiring different, stepped etched depths as taught in de Besten et al. in the waveguides in these transition regions.